Over the past few years, several "perpendicular recording media" so called because the recording medium is magnetized perpendicular to the surface of its magnetizable thin film, have been proposed for increasing the density of magnetic recording. Japanese Patent Publication No. 58-91 discloses one perpendicular recording medium comprising a polyimide substrate and a two-layer magnetic thin film thereon, one layer on the substrate being a low-coercive force layer of molybdenum/iron/nickel and the other layer thereon being a magnetic recording layer of cobalt/chromium. Magnetic thin films of this two-layer structure type provide a number of advantages. Since the magnetic circuit on the back surface of the perpendicularly magnetizable cobalt/chromium film is partially closed by the high permeability, low coercive force magnetic layer, magnetic deterioration is minimized and residual magnetization is enhanced.
Magnetic thin films having the overall desired properties can be obtained by making the magnetic thin film a multilayer structure consisting of layers with different properties.
It has hitherto been the practice to form a magnetic thin film having such a two-layer structure by heating the non-magnetic substrate to temperatures above room temperature (250.degree. C. in examples described in Japanese Patent Publication No. 58-91), forming a low coercive force, high permeability magnetic layer thereon by sputtering, and further forming a magnetic recording layer of cobalt/chromium thereon by sputtering. The low coercive force, high permeability magnetic layer formed by this process is a polycrystalline structure in which various crystal faces including (110), (111), and (100) are exposed on its surface. If a magnetic recording layer destined for perpendicular magnetization is formed on the polycrystalline high-permeability magnetic layer, the irregular crystal faces on the surface of the underlying magnetic layer will affect the overlying magnetic recording layer being formed particularly at the initial stage of the process, resulting in local variations of the crystal growth of the magnetic recording film. More specifically, crystal growth on (111) faces is promoted, but growth on other faces is retarded. This results in the disordered crystal growth and orientation of the magnetic recording layer with deteriorated surface planarity.